US5145924A

US5145924A - Free radical polymerization of styrene monomer

Info

Publication number: US5145924A
Application number: US07/786,160
Authority: US
Inventors: Eric J. Shero; James J. O'Brien; Duane B. Priddy
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1991-02-06
Filing date: 1991-10-31
Publication date: 1992-09-08
Anticipated expiration: 2011-02-06

Abstract

High molecular weight polymers of vinyl aromatic monomers are prepared by free radical polymerization in the presence of 5 to 5000 ppm of a soluble organic acid having pKa from 0.5 to 2.5.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of application Ser. No. 07/651,072, filed Feb. 6, 1991, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a process for polymerization of vinyl aromatic monomers. More particularly the present invention relates to an improved process for the free radical polymerization of vinyl aromatic monomers to make high molecular weight polymers.

Currently, production of high molecular weight vinyl aromatic polymers, particularly polymers having weight average molecular weights (M_w) of greater than 300,000, is best performed by the use of anionic polymerization techniques. This is due to the extremely slow polymerization rates required to make high molecular weight vinyl aromatic polymers using free radical chemistry. Disadvantageously however, anionic polymerization processes require expensive anionic initiators and tend to produce discolored products due to the presence of residual lithium-containing salts. In addition, anionic processes utilize different equipment than free radical processes. Consequently commercial producers of vinyl aromatic polymers by means of free radical chemistry must invest in anionic polymerization equipment in order to prepare very high molecular weight polymers. Finally, anionic polymerization cannot be employed to prepare many copolymeric products. In many cases the monomer is not amenable to anionic polymerization. In other cases block copolymers are formed due to unequal reactivities of the comonomers.

It would be desirable if it were possible to produce high molecular weight polyvinyl aromatic resins utilizing free radical polymerization equipment while obtaining rates that are commercially practical. Thus it is to the attainment of the preparation of such high molecular weight polymers via free radical polymerization techniques that the present invention is directed.

According to the present invention there is provided a process for free radical polymerization of a vinyl aromatic monomer to prepare a high molecular weight polymer characterized in that the polymerization is conducted in the presence of from 5 to 5000 parts per million (ppm) of a soluble organic acid having a pKa from 0.5 to 2.5, at 25° C. It has been surprisingly discovered that in the presence of such an amount of these acids the free radical polymerization rate is substantially increased thereby allowing the attainment of high molecular weight polymers in reasonable reaction times.

The vinyl aromatic monomers usefully employed according to the present process include styrene, ring alkyl substituted styrene, particularly C_1-4 alkyl and especially methyl ring substituted styrenes and α-methylstyrene. A preferred monomer is styrene. The polymerization can also include a comonomer to prepare vinyl aromatic copolymers. The comonomer must be noninterfering with the acid. Examples include (meth)acrylonitrile, (meth)acrylic acid and C_1-4 alkyl esters thereof, N-C_1-4 alkyl maleimide, N-phenyl maleimide, maleic anhydride, etc. In addition the polymerization may be conducted in the presence of predissolved elastomer to prepare impact modified, grafted rubber containing products.

By the term "soluble" is meant that the acid is sufficiently soluble in the reaction mixture to achieve the indicated concentration of organic acid. Preferred organic acids are miscible with neat styrene monomer. Suitable organic acids include the C_1-20 alkyl and aryl substituted sulfonic and phosphonic acids. Examples include methane sulfonic acid, toluene sulfonic acid, camphorsulfonic acid, napthalene sulfonic acid, methyl phosphonic acid, phenyl phosphonic acid, etc. Strong acids, i.e., organic acids having a pKa less than 0.5, are not desired due to increased incidence of cationic polymerization as opposed to the desired free radical initiation. Preferred acids have pKa from 1.0 to 2.0. A preferred organic acid is camphorsulfonic acid.

Similarly it has been discovered that at increased concentrations of organic acid, cationic polymerization becomes prevalent. Generally, acids with higher pK, i.e. weaker acids, may be employed in higher concentration without detrimental effect. Stronger acids are employed in relatively smaller concentration. Cationic polymerization is undesirable because it results in extremely low molecular weight oligomer formation. Even small quantities of such low molecular weight product would significantly reduce the molecular weight average of the resulting product. Most preferred are amounts of organic acid from 50 to 5000 ppm. The amount of acid is measured with respect to the molar quantity of vinyl aromatic monomer.

A free radical initiator may be employed to further improve the rate of free radical initiation. Suitable catalysts include organic peroxides or other well known free radical initiators.

The monomer may be polymerized in bulk, i.e., in the absence of a diluent, or in the presence of a diluent, i.e., in solution. Suitable diluents include toluene, ethylbenzene, and other noninterfering organic liquids. Preferably the reaction is conducted under bulk polymerization conditions.

The polymerization rate according to the present process is substantially increased and the resulting product has substantially increased molecular weight compared to products prepared by free radical polymerization in the absence of an organic acid. However, because the product has increased molecular weight, the conversion rate is less at higher acid concentrations compared to lower acid concentrations. That is, the higher molecular weight polymers require longer reaction times despite incrementally faster polymerization rates. Preferred polymer product has a molecular weight (Mw) from 300,00 to 1,000,000, more preferably 500,000 to 800,000, based on a polystyrene standard as measured by size exclusion chromatography.

The products are employed in applications where high molecular weight vinylaromatic polymers have previously found suitable uses. Particularly preferred are molding polymers comprising the presently prepared polymeric products. The product may be blended with other ingredients such as mold release additives, lubricants, colorants, ignition resistant additives, impact modifiers, glass fibers, as well as other resins such as polyvinylaromatic resins having different molecular weights, polyphenylene oxides, polycarbonates, elastomeric copolymers such as styrene-butadiene block copolymers, polybutadiene, etc.

Having described the invention the following examples are provided as further illustrative and are not to be construed as limiting.

EXAMPLE 1-3

Aliquots of styrene monomer which was purified by degassing and contacting with alumina were placed in glass tubes. To each tube was added an amount of methane sulfonic acid further identified in Table 1. The tubes were sealed under vacuum and place in an oil bath at 150° C. for 1 hour. The tubes the weight average molecular weight of the polystyrene in each tube measured using size exclusion chromatography. Results are contained in Table 1.

              TABLE 1                                                     
______________________________________                                    
Run       Methane Sulfonic Acid (ppm)                                     
                             Mw                                           
______________________________________                                    
*          0                 270,000                                      
1         100                460,000                                      
2         300                580,000                                      
3         500                830,000                                      
______________________________________                                    
 *comparative

It may be seen that significant increase in molecular weight is observed upon addition of small quantities of the acid.

EXAMPLE 4-8

The reaction conditions of Examples 1-3 were substantially repeated employing various concentrations of camphorsulfonic acid (CSA) at a reaction temperature of 140° C. Weight average molecular weight of the resulting polystyrene and conversion rates are provided in Table II.

              TABLE II                                                    
______________________________________                                    
                       Conversion                                         
Run    Amt. CSA ppm    Rate (%/hr)                                        
                                  Mw                                      
______________________________________                                    
*       0              43         280000                                  
4      100             25         400000                                  
5      250             18         510000                                  
6      500             14         650000                                  
7      750             12         700000                                  
8      1000            10         710000                                  
______________________________________                                    
 *comparative

EXAMPLES 9-10

The reaction conditions of Examples 1-3 are substantially repeated utilizing phenylphosphonic acid (PPA) at a reaction temperature of 150° C. Weight average molecular weight of the resulting polystyrene and conversion rates are provided in Table III.

              TABLE III                                                   
______________________________________                                    
        Amt. PPa      Conversion                                          
Run     (wt. %)       Rate (%/hr)                                         
                                 Mw                                       
______________________________________                                    
*       0             50         240000                                   
9       0.1           40         280000                                   
10      0.5           25         430000                                   
______________________________________                                    
 *comparative

Claims

What is claimed is:

1. A process for free radical polymerization of styrene to prepare high molecular weight polystyrene comprising conducting the polymerization in the presence of from 5 to 5000 ppm of a soluble organic acid having pKa from 0.5 to 2.5 at 25° C.

2. A process according to claim 1 wherein the organic acid is methane-sulfonic acid.

3. A process according to claim 1 wherein the organic acid is camphorsulfonic acid.

4. A process according to claim 1 wherein the polystyrene has a Mw from 300,000 to 1,000,000.